482 MANUAL OF PHYSIOLOGY. 



usually allowed to rest on one leg while the other lightly touches 

 the ground to form a kind of stay and relieve the muscles which 

 surround the supporting ankle from too great an effort of bal- 

 ancing. At the same time the knee is extended and the pelvis 

 becomes somewhat oblique so as to bring it more directly over 

 the head of the femur. In ordinary easy standing, the joints are 

 not commonly kept locked by the tension of the ligamentous 

 structure, but their position is constantly being very slightly 

 altered so as to vary the muscles employed in preserving the bal- 

 ance and thus to prevent fatigue. 



The joints most exercised in the erect posture are the follow- 

 ing : 



1. The ankle has to support the weight of the entire body, 

 while the joint is neither flexed nor extended to its utmost, and 

 cannot be fixed in this position by ligamentous arrangements. 

 The foot, being placed on the ground, resting on the heel and the 

 balls of the great and little toes, is supported in an arch-like form 

 by strong, though elastic, ligaments, which allow but little motion 

 in the numerous joints. The bones of the leg can move in the 

 freest way, backward or forward, around the articular surface 

 of the astragalus, which forms the roller of the hinge, any lateral 

 motion of which is prevented by the malleoli. The line passing 

 through the centre of gravity of the body generally falls slightly 

 in front of the axis of rotation of the ankle joint, so that the 

 entire body tends to fall forward at the ankles. This tendency 

 is checked by the powerful calf muscles, which, attached to the 

 calcaneum by means of the strong tendo-Achillis, keep the parts 

 in such a position that an exact balance is nearly constantly 

 kept up. 



2. The knee joint, when completely extended, requires no mus- 

 cular action to prevent it from bending, because the line of 

 gravity then passes in front of the axis of rotation, and the 

 weight of the body tends to bend the knee backward. This is 

 impossible on account of the powerful ligaments which exert 

 their traction behind the axis of rotation. Commonly, however, 

 these ligaments are not put on the stretch in this way, but the 

 joint is held, by muscular power, in such a position that the line 



